Unraveling Epitaxial Habits in the NaLnF4 System for Color Multiplexing at the Single‐Particle Level

We report an epitaxial growth technique for scalable production of hybrid sodium rare‐earth fluoride (NaLnF₄) microcrystals, including NaYF₄, NaYbF₄, and NaLuF₄ material systems. The single crystalline nature of the as‐synthesized products makes them strong upconversion emission. The freedom of combining a lanthanide activator (Er³⁺ or Tm³⁺) with a sensitizer (Yb³⁺) at various doping concentrations readily gives access to color multiplexing at the single‐particle level. Our kinetic and thermodynamic investigations on the epitaxial growth of core–shell microcrystals using NaLnF₄ particle seeds suggest that within a certain size regime it is plausible to exert precise control over shell thickness and growth orientation under hydrothermal conditions.     

分子束外延PbTe单晶薄膜的反常拉曼光谱研究

采用分子束外延(MBE)方法在BaF₂(111)衬底上生长了高质量的PbTe单晶薄膜， 拉曼光谱测量观察到了表面氧化物的振动模、布里渊区中心(q≈0)纵光学(LO)声子振动模以 及声子-等离子激元耦合模振动.随着显微拉曼光谱仪激光光斑聚焦深度的改变，各拉曼散射 峰的峰位、积分强度、半高宽等都表现出不同的变化趋势. 随着激光光斑聚焦位置从样品表 面上方3μm处变化到表面下方3μm处，PbTe外延薄膜的LO声子频率从119cm^-1移 动到124cm^{-1关键词： PbTe外延薄膜 拉曼散射 纵光学声子}     

AlAsSb的外延再生长对InAs/GaSb红外探测器暗电流抑制效果研究

采用分子束外延(MBE)技术外延再生长AlAsSb,对InAs/GaSb Ⅱ类超晶格(T2SLs)长波红外探测器的表面缺陷进行钝化,实现了暗电流的显著降低.首先,研究了湿法腐蚀浅台面的最佳腐蚀液配比,获得了低横向腐蚀、光滑的侧壁以及均匀的腐蚀界面.随后,使用AlAsSb/AlAs/GaAs、AlAsSb/GaSb两种不同的外延再生长组合和单一的SiO₂薄层分别对刻蚀后的台面进行钝化,同时保留一个没有钝化的样品作为对照,最后对4种不同钝化条件下探测器的暗电流特性进行了测量.结果发现,使用AlAsSb/GaSb外延再生长钝化层的器件暗电流得到了进一步降低,使用SiO₂钝化层的效果次之,而使用AlAsSb/AlAs/GaAs外延再生长钝化层的性能相对较差.上述结果表明,外延再生长钝化技术是降低长波红外探测器暗电流的一种有效方法.     

Energetics of Ge nucleation on SiO2 and implications for selective epitaxial growth

We have measured the time evolution of Ge nucleation density on SiO₂ over a temperature range of 673–973 K and deposition rates from 5.1 × 10¹³ atoms/cm² s (5 ML/min) to 6.9 × 10¹⁴ atoms/cm² s (65 ML/min) during molecular beam epitaxy. The governing equations from mean-field theory that describe surface energetics and saturation nucleation density are used to determine the size and binding energy of the critical Ge nucleus and the activation energy for Ge surface diffusion on SiO₂. The critical nucleus size is found to be a single Ge atom over substrate temperatures from 673 to 773 K, whereas a three-atom nucleus is found to be the critical size over substrate temperatures from 773 to 973 K. We have previously reported 0.44 ± 0.03 eV for the Ge desorption activation energy from SiO₂. This value, in conjunction with the saturation nucleation density as a function of substrate temperature, is used to determine that the activation energy for surface diffusion is 0.24 ± 0.05 eV, and the binding energy of the three-atom nucleus is 3.7 ± 0.1 eV. The values of the activation energy for desorption and surface diffusion are in good agreement with previous experiments of metals and semiconductors on insulating substrates. The small desorption and surface diffusion activation barriers predict that selective growth occurring on window-patterned samples is by direct impingement of Ge onto Si and ready desorption of Ge from SiO₂. This prediction is confirmed by the small integral condensation coefficient for Ge on SiO₂ and two key observations of nucleation behavior on the window-patterned samples. The first observation is the lack of nucleation exclusion zones around the windows, and second is the independence of the random Ge nucleation density on patterned versus unpatterned oxide surfaces. We also present the Ge nucleation density as a function of substrate temperature and deposition rate to demarcate selective growth conditions for Ge on Si with a window-patterned SiO₂ mask.     

Properties of n-Ge epilayer on Si substrate with in-situ doping technology

The properties of n-Ge epilayer deposited on Si substrate with in-situ doping technology in a cold-wall ultrahigh vacuum chemical vapor deposition(UHVCVD) system are investigated.The growth temperature of ~500℃ is optimal for the n-Ge growth in our equipment with a phosphorus concentration of ~10~(18)cm~(-3).In the n-Ge epilayer,the depth profile of phosphorus concentration is box-shaped and the tensile strain of 0.12% confirmed by x-ray diffraction measurement is introduced which results in the red shift of the photoluminescence.The enhancements of photoluminescence intensity with the increase of the doping concentration are observed,which is consistent with the modeling of the spontaneous emission spectrum for direct transition of Ge.The results are of significance for guiding the growth of n-Ge epilayer with in-situ doping technology.     

Phase transition-induced superstructures of β-Sn films with atomic-scale thickness

The ultrathin β-Sn(001) films have attracted tremendous attention owing to its topological superconductivity(TSC),which hosts Majorana bound state(MBSs) for quantum computation.Recently,β-Sn(001) thin films have been successfully fabricated via phase transition engineering.However,the understanding of structural phase transition of β-Sn(001)thin films is still elusive.Here,we report the direct growth of ultrathin β-Sn(001) films epitaxially on the highly oriented pyrolytic graphite(HOPG) substrate and the characterization of intricate structural-transition-induced superstructures.The morphology was obtained by using atomic force microscopy(AFM) and low-temperature scanning tunneling microscopy(STM),indicating a structure-related bilayer-by-bilayer growth mode.The ultrathin β-Sn film was made of multiple domains with various superstructures.Both high-symmetric and distorted superstructures were observed in the atomic-resolution STM images of these domains.The formation mechanism of these superstructures was further discussed based on the structural phase transition of β to α-Sn at the atomic-scale thickness.Our work not only brings a deep understanding of the structural phase transition of Sn film at the two-dimensional limit,but also paves a way to investigate their structure-sensitive topological properties.     

HgTe/CdS/ZnS多壳层量子点的制备与表征

采用一种简单的方法合成HgTe/CdS/ZnS多壳层量子点。首先,以1-硫代甘油为稳定剂,在水相溶液中制备出HgTe核量子点;然后,采用外延生长法依次在HgTe核量子点表面包覆CdS和ZnS壳层,合成出最终具有稳定近红外发光的HgTe/CdS/ZnS多壳层量子点。该合成方法仅需3个步骤,具有操作简单、成本低廉的优点。实验结果显示,当反应温度为90 ℃、反应溶液pH为11.0、反应加热回流时间为4 min时,HgTe/CdS/ZnS多壳层量子点具有最高荧光量子产率36%。     

Anisotropic transport and optical properties of superconducting (La1-xSrx)2CuO4 single crystal thin films

An experimental study on the resistivity, Hall coefficient, upper critical field, and optical reflection and transmission characteristics is described for single-crystalline superconducting (La_1-xSr_x)₂CuO₄ thin films epitaxially grown on SrTiO₃ single crystal substrates by rf magnetron sputtering. These properties exhibit pronounced anisotropy between the c-axis direction and the Cu-O plane. The characteristics are consistently explained from the point of view of the two-diemensional (2D) electronic structure. The most noteworthy implication of the experimental results is that the transport along the c-axis is far from an itinerant picture. This leads to the idea that the superconductivity in the series of cuprate materials is caused by the 2D electronic system arising from the Cu-O planes, and some interactions between them, which is not possibly due to itinerant carriers.     

Experimental investigation of different configurations for the seeded growth of SiC crystals via a VLS mechanism

The growth of SiC crystals or epilayers from the liquid phase has already been reported for many years. Even if the resulting material can be of very high structural quality and the possibility to close micropipes was demonstrated, handling the liquid phase still is a challenge. Moreover, it is highly difficult to stabilize the C dissolution front and then to stabilize the growth front over a long growth time. Based on the Vapour‐Liquid‐Solid (VLS) mechanism, we present a new configuration for the growth of SiC single crystal which should allow first to simplify the liquid handling at high temperature and second to precisely control the crystal growth front. The process consists in a modified top and bottom seeded solution growth method, in which the liquid is held under electromagnetic levitation and fed from the gas phase. 3C‐SiC crystals exhibiting well‐faceted morphology were successfully obtained at 1100‐1200 °C with exceptional growth rates, varying from 1 to 1.5 mm/h in Ti‐Si melt. It was shown that the nucleation density decreases simultaneously with increasing propane partial pressure. At 1200‐1400 °C, thick homoepitaxial 6H‐SiC layers were successfully obtained in Co‐Si and Ti‐Si melts, with growth rate up to 200 µm/h. Large terraces with smooth surfaces are observed suggesting a layer by layer growth mode, and the influence of the system pressure was demonstrated. It was shown that the terrace size decrease simultaneously with increasing propane partial pressure which suggests the beginning of a two dimensional to three dimensional growth mode transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microhardness of 6H-SiC Epitaxial Layers Grown by Sublimation

Knoop microhardness of 6H-SiC layers grown by sublimation epitaxy was investigated. The microhardness-load curves for all of the samples were measured and then used to extract the load-independent microhardness values. The relationships of these values to the growth time and growth rate were studied. The microhardness-depth profiles indicated that the layer/substrate interface region had a microhardness value that differed significantly from that of both the epi-layer and the substrate.